Ice Grain Size Distribution: Differences Between Jovian and Saturnian Icy Satellites From Galileo and Cassini Measurements

Details Ice Grain Size Distribution: Differences Between Jovian and Saturnian Icy Satellites From Galileo and Cassini Measurements Ice Grain Size Distribution: Differences Between Jovian and Saturnian Icy Satellites From Galileo and Cassini Measurements

Dec 2005

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2005agufm.p11b0124h&link_type=abstract

American Geophysical Union, Fall Meeting 2005, abstract #P11B-0124

Physics

5422 Ices, 5464 Remote Sensing, 5470 Surface Materials And Properties, 6280 Saturnian Satellites

Scientific paper

Most of the satellites of the outer solar system have surfaces substantially covered by water ice, whose near-infrared (1-5 μm) spectrum has absorption bands near 1.05, 1.25, 1.5, and 2.0 μm, and a strong broad absorption beyond 3 μm. For fine-grained ice, a reflection peak near 3.6 μm occurs. For grain sizes larger than a micrometer, a Fresnel reflection peak appears near 3.1 μm. The reflection spectrum of the icy Galilean satellites of Jupiter (Callisto, Ganymede and Europa), as returned by the Galileo Near Infrared Mapping Spectrometer, are well fit by ice models using a single, or at most a closely-spaced bimodal, grain size in the range from 10 to several 100 μm, depending on satellite and location. These spectra all have a strong 3.1-μm peak. The reflection spectrum of the Saturnian icy satellites Mimas, Tethys, Rhea, Enceladus, Dione, and the icy hemisphere of Iapetus was returned by the Cassini Visual and Infrared Mapping Spectrometer instrument. They are are very different, consistent with grain sizes ranging from 1 to 5 μm, comparable to telescopic observations over the last two decades. The 3.1-μm reflection peak is also often muted or indistinct and is generally small at high phase angles. This behavior can be modeled as a coarse-sized distribution covered by purely absorbing sub-micron grains. We know that the radiation environment is much stronger in the Jovian system, and this is perhaps the cause of the coarse-grained (0.5-1 mm) ice seen on the trailing sides of Europa and Ganymede. All the Galilean satellites surfaces show recent evolution, including resurfacing on Europa and volatile-driven mass wasting on Callisto, and can experience thermal grain coarsening more readily at the higher Jovian temperatures. The grain sizes on the Saturnian satellites may indicate much older surfaces in a different thermal and radiation environment that results in an undisturbed deposit of micrometeorite gardened micron to sub-micron grains.

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